Horton

Re: Improve LWLock tranche name visibility across backends

Nathan Bossart <nathandbossart@gmail.com>

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From: Nathan Bossart <nathandbossart@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Sami Imseih <samimseih@gmail.com>, Rahila Syed <rahilasyed90@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2025-08-19T18:44:39Z
Lists: pgsql-hackers 
On Tue, Aug 19, 2025 at 02:37:19PM -0400, Andres Freund wrote:
> On 2025-08-19 13:31:35 -0500, Nathan Bossart wrote:
>> On Tue, Aug 19, 2025 at 02:06:50PM -0400, Andres Freund wrote:
>> > Possibly stupid question - is it really worth having a dynamic structure here?
>> > The number of tranches is strictly bound, it seems like it'd be simpler to
>> > have an array of tranch nmes in shared memory.
>> 
>> Tranches can be allocated post-startup with LWLockNewTrancheId() (e.g.,
>> autoprewarm).
> 
> Sure, but we don't need to support a large number of tranches. Just make it,
> idk, 128 entries long. Adding a dynamically allocated dsm to every server
> seems like a waste - ever shared mapping makes fork / exit slower...

The other issue is that there's presently no limit on the length of a
tranche name registered via LWLockRegisterTranche().  Life would become
much simpler if we're willing to put a limit on both that and the number of
tranches, but thus far we've been trying to avoid it.

-- 
nathan

Commits

  1. test_dsa: Avoid leaking LWLock tranches.

  2. Teach DSM registry to ERROR if attaching to an uninitialized entry.

  3. Add a test harness for the LWLock tranche code.

  4. Revert recent change to RequestNamedLWLockTranche().

  5. Move dynamically-allocated LWLock tranche names to shared memory.

  6. Prepare DSM registry for upcoming changes to LWLock tranche names.

  7. Add GetNamedDSA() and GetNamedDSHash().